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is stabilized by the presence of the nitro groups, TNT is recalcitrant. Although it can be mineralized, biodegradation usually involves reduction of one or more of the nitro groups to an amine, which then reacts with a nitro group on an adjacent molecule (Figure 13.14). A series of such reactions leads to the disappearance of TNT through the formation of a polymer. The environmental significance of such polymers is still not thoroughly understood, as there is concern that the monomers may be released under some conditions.

Polynuclear aromatic hydrocarbons (PAHs or PNAs) are compounds containing two or more condensed (fused together) aromatic rings (Figure 13.15). They are common constituents of coal, oil, and creosote (a wood preservative) and are also products of incomplete combustion. They include the first known human carcinogen, benzo[a]pyrene naphthalene naphthalene anthracene anthracene phenanthrene phenanthrene pyrene chrysene benzo(a)pyrene pyrene chrysene benzo(a)pyrene fluorene fluorene fluoranthene fluoranthene o dibenzofuran o dibenzodioxin biphenyl

Figure 13.15 Polynuclear aromatic hydrocarbons and related compounds.

(BAP), which was found to be a cause (because of its presence in soot) of testicular cancer in English chimney sweeps.

Generally, the PAHs with two or three rings are readily biodegradable, at least under aerobic conditions. The four-ring compounds pyrene and chrysene are also usually mineralized, although chrysene more slowly. However, there usually seems to be little mineralization (with occasional exceptions) of the higher-molecular-weight PAHs. BAP does appear to be cometabolized to some extent. Probably as a result of initial cometabolic activity, it also appears to be incorporated into the soil humic materials. These complex, irregular soil organic polymers, such as humic acid (Figure 13.16), include phenolics, het-erocyclics, sugars, and amino acid residues as building blocks, some probably derived from lignin degradation. Once a BAP or other PAH residue is incorporated into the polymer, it probably cannot be distinguished from other subunits, and thus is no longer expected to be of concern.

Over long periods of time, humic materials buried in anaerobic layers may be reduced further. Eventually, after millions of years, a combination of microbial degradation and abiotic processes at elevated temperatures and pressures has led to coal and oil formation. These organic materials have thus been held out of the active cycling of carbon for

Figure 13.16 Example of a possible humic acid chemical structure.

100 million years or more. The rapidly increasing use of these fossil fuels over the last century is responsible for an increase in atmospheric CO2 as this carbon reenters active cycling.

Note that although humic materials are not usually referred to as recalcitrant, they do not biodegrade rapidly. Rather, they are considered to be stable or to change only slowly. Stabilization of wastes thus refers to elimination of the readily degradable components through both mineralization and conversion to high molecular weight products such as humics.

Greenhouse Gases In most cases, a desired major product of biodegradation is carbon dioxide. However, it is now recognized that atmospheric CO2 concentrations have increased dramatically during the industrial age, from around 280 ppm prior to the late eighteenth century to 380 ppmv today. Most of this increase is from the combustion of fossil fuels, although some other human activities, such as deforestation, loss of soil organic matter, and draining of wetlands, may also have had a small role. The higher concentration of CO2 acts to capture more of the infrared energy radiated from Earth's surface. This phenomenon, often referred to as the greenhouse effect, has contributed to an increase in average temperature worldwide, or global warming. Thus, some thought is now being given to slowing mineralization of certain organic materials and perhaps to rebuilding some stores of humic materials in soil, for example. This may be an area in which environmental engineers and scientists can make contributions in the future.

Carbon dioxide is the most abundant, but it is not the only greenhouse gas. In fact, methane has about 26 times the heat-trapping capacity of CO2. Additionally, its concentration in the atmosphere has increased even more (proportionally) than that of carbon dioxide, from about 0.7 to almost 1.8 ppm. Much of this increase is from fossil-fuel extraction and use, but other major anthropogenic sources include cattle herds (digestive tracts), rice fields (flooded soils), and solid waste landfills. Even landfills with gas collection systems produce fugitive emissions. This is another area in which environmental professionals can expect to be called upon to help control greenhouse gas releases.

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